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More Debugging of the PDP-11 H7861 PSU (I fixed it!)

Posted <2017-10-31 Tue 20:23> by Aaron S. Jackson.

In a previous post I mentioned that the 12V rail in my PDP-11 was dead. It turned out not to be the MJE15030 transistor. So, once again I've been trying to figure out what the problem is.

The oscillator for the 5V is giving about 65KHz, which is what you would expect from a switch mode power supply. On the 12V side, there is nothing! So, it looks like I have found the area containing the problem.

I'll probably buy a replacement 555 timer. I have tested surrounding components and they look alright.

In the mean time, I've emailed classiccmp to see if they have any thoughts.

Exciting! I hope it is as simple as replacing the 555.

<2017-11-01 Wed 18:15> I am so happy. It was a dead 555 and now it is repaired!

I made a short video about this repair:

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